Rodents on tropical land-bridge islands

The results are reported of a survey of rodents on 10 forested land-bridge islands ranging in size from 0.2 to 350 ha in the state of Bolivar, Venezuela. The islands were contained within a lake formed c. 12 years before the study by the damming of the Caroni River for hydroelectric power. Rodents were sampled on each island by live-trapping along transects that sampled all available habitat types on each island, and microhabitat structure was measured at each trap station. A total of 674 captures of 359 individuals of six species of rodents was recorded. Species composition changed from the largest to the smallest islands, and small and medium islands (0.2-11 ha) displayed the typical effects of insularity, with fewer species and increased abundances and biomass. The largest island (3 50 ha) seemed to function more like a mainland. Most species were associated with a suite of microhabitat variables. It is suggested that release from top-down control by predators was responsible for higher abundances and biomass on the smaller islands and that predators moving between large islands and other nearby landmasses help maintain a mainland community structure on large islands. However, changes in species composition on smaller islands may be the result of patchy occurrences of some species before isolation, changes in microhabitat structure following isolation, and species-specific microhabitat requirements.